Memory problems and old photographs

It has been said on many occasions that it's good to show people with memory problems, old photographs taken when they were younger.

But I have heard other people argue the opposite, 

The reason being that the photographs could trigger old happy memories, which could cause the person to be fixated with that period, and may well then forget what is happening in reality.

I know that when so one shows me an old photograph, I try to remember who is there, and wonder  what happened to them, if they are people I have not seen for a long time

But could this cause extra problems for those with bad memory problems or dementia, and could it confuse these people even more. Could they then go on to think that all of these people are still alive, even though they could have died many years ago.

They could also cause upset, because these photographs may trigger an unhappy experience, which no one else knows about. 

I know from personal experience that old photographs can cause the imagination to go into overdrive, trying to remember who was there, where they lived, their ages, their families etc.

So is this a good idea or a bad one, I suppose it all depends on the individuals, because we are all so very different.